I0-6 Tesla

Galileo Galilei discovered Io in 1610, in the same week that he discovered Ganymede, Europa, and Callisto. Io orbits Jupiter four times for every time Ganymede orbits Jupiter once, as well as four times for every two times that Europa orbits the planet. This arrangement made it possible to estimate the moons' masses long before spacecraft visited them. Io orbits Jupiter at just 5.9 Jupiter radii. From Io, Jupiter would look 40 times larger than the Moon does from Earth.The Voyager 1 mission first discovered Io's volcanism in March 1979. Galileo, the next mission slated to visit the outer solar system, was already planned. The discovery of active volcanoes was so startling that Galileo was promptly redesigned to include an instrument for infrared mapping, so that the surface temperatures of the moon could be detected. Galileo orbited Jupiter 34 times and took 600 images of Io, some at a resolution of just 10 meters per pixel. Gas and dust plumes 500 miles (800 km) high issue from the volcanoes.These are truly gigantic plumes; the plume from a recent large eruption in the Kamchatka peninsula on Earth was only three miles (5 km) high (the Kamchatka peninsula reaches south from the eastern Siberia). One plume that erupted from Io, shown in the figure on page 72, created fallout that covers an area the size of Alaska. The vent is a dark spot just north of the triangular-shaped plateau (right center).To the left, the surface is covered by colorful lava flows rich in sulfur.

In one image two volcanic plumes were captured on Io (see figure on next page). One plume was captured on the bright limb or

The plume from Ios volcano Pele rises 190 miles (300 km) above the surface in an umbrella-like shape, and its fallout covers an area the size ofAlaska. (NASA/JPL)

The plume from Ios volcano Pele rises 190 miles (300 km) above the surface in an umbrella-like shape, and its fallout covers an area the size ofAlaska. (NASA/JPL)

edge of the moon, erupting over the caldera named Pillan patera. Pillan patera's plume is 85 miles (140 km) high and has also been seen by the Hubble Space Telescope. The second plume, Prometheus, is seen near the terminator (the boundary between day and night). The shadow of the airborne plume can be seen extending to the right of the eruption vent. (The vent is near the center of the bright and dark rings). The Prometheus plume can be seen in every Galileo image viewing the correct part of Io, as well as by every Voyager image of that region. It is possible therefore that this plume has been continuously active for more than 18 years.

The lavas produced by the voluminous and possibly continuous eruptions on Io create rivers of flowing lava on the moon's surface, such as those shown in the figure on page 74. The lava channel is dark and runs to the right from the dark patera (Emakong patera) at the left of this mosaic. As on Earth, lava cools as it flows along the surface, and so maintaining surface lava channels hundreds of kilometers long is a problem. Close inspection of this and other lava channels on Io show that they are partially roofed over:The cooling lava has formed a crust that connects to the sides of the channel, forming a roof under which lava can flow and retain its heat through insulation.

Io's surface is sulfur-covered from volcanic eruptions that spray different compositions onto its surface, creating bright regions colored white, yellow, orange, red, green, and black, depending on the sulfur content and chemical composition. The different sulfur compounds required to make the many colors of Io's surface can be

Both Pillan and Prometheus paterae are erupting in this image, as described in the text. (NASA/JPL)

This mosaic sets several high-resolution images (100 feet, or 30 m, per picture element) into the context of lower-resolution images (490 feet, or 150 m, per picture element). Galileo took the high-resolution ones during a close flyby of lo on October 15, 2001. (NASA/JPL/Ga/i'/eo)

created by differing amounts of heat. Shown in the lower color insert on page C-5, the small SO plumes make irregular or circular rings 30 to 220 miles (50 to 350 km) in diameter that are white, yellow, or black. Giant sulfur plumes make oval markings on Io's surface, elongated north to south, 220 to 500 miles (350 to 800 km) in length, colored red or orange. These are huge extents of eruption: On the Earth, an eruption 500 miles (800 km) in length would reach from Boston to Washington, D.C. Some of these giant plumes erupt fairly continuously, while others seem to be single events.

Because Io orbits within the most intense part of Jupiter's magnetosphere, Jupiter's magnetic field is strong enough to pull atmospheric material and volcanic gases away from Io and into orbit around Jupiter (Io, surprisingly, has its own magnetic field, about 10-6Tesla, but it is much weaker than Jupiter's). About one ton (1,000 kg) of sulfur, atmospheric sodium, potassium, and chlorine is pulled off Io every second, and these ions form a cloud around the planet as well as a doughnut-shaped cloud of ions around Jupiter, in Io's orbit.These ions create spectacular auroras by interacting with Jupiter's magnetic field. The source of the sodium, potassium, and chlorine is not understood, however, because these volatile elements should have been depleted earlier in Io's history. The movement of these electrically charged ions through Jupiter's magnetic field creates the effect of a electrical generator: Every magnetic field can create a linked electrical field, and, in

This mosaic sets several high-resolution images (100 feet, or 30 m, per picture element) into the context of lower-resolution images (490 feet, or 150 m, per picture element). Galileo took the high-resolution ones during a close flyby of lo on October 15, 2001. (NASA/JPL/Ga/i'/eo)

this case, Io and its charged particles become the electrical field. Io commonly develops 400,000 volts across its diameter and generates an electric current of 3 million amperes that flows along the magnetic field to Jupiter's ionosphere.

At first volcanism was thought to be entirely sulfuric, but temperatures now show differently. Sulfur eruptions occur at temperatures less than about 800°F (430°C), while basaltic lava eruptions on Earth occur at temperatures of about 2,000 to 2,700°F (1,100 to 1,500°C). The Voyager 1 mission measured eruption temperatures only up to about 750°F (400°C), consistent with sulfur-rich eruptions, but the Galileo mission detected eruptions at temperatures greater than 800°F (430°C), and even saw one eruption in 1997 at Pillan patera with lava temperatures of 2,900 ± 45°F (1,600 ± 25°C). Four eruptive centers have been measured at above 2,060°F (1,100°C): Pillan, Masubi, Pele, and Surt paterae. These high eruptive temperatures show that silicate liquids must be involved in some of the eruptions, while others with lower temperatures consist mainly of sulfur compounds. Most volcanic eruptions on Io are at about 2,600°F (1,430°C), about the temperature of hot silicate magmas on Earth. The hottest eruptions on Io, at about 2,900°F (1,600°C), are hotter than any volcanic eruptions on Earth now, and may be hotter than any eruptions that ever occurred on Earth. This could be due to two things: Either lava is heated well above the temperature it requires to become molten (superheated), or the material being melted requires very high temperatures to melt, higher than the mantle on Earth requires.The images in the upper color insert on page C-6 from the Galileo mission show changes in the largest active lava flows in the solar system, in the 190-mile (300-km)-long Amirani lava field on Io. Researchers have identified 23 distinct new flows by comparing the two images taken 134 days apart, on October 11, 1999, and February 22, 2000. Individual flows within it are each several kilometers or miles long, which is about the size of the entire active eruption on Kilauea, Hawaii. The new lava flows at Amirani identified in these images cover about 240 square miles (620 km2) in total over the five months of observation. In comparison, Kilauea covered only about four square miles (10 km2) in the same time, and Io's Prometheus lava flow field covered about 24 square miles (60 km2).

The largest eruptive center on Io is called Loki patera. Loki patera is 140 miles (220 km) long, with an area of 11,600 square miles

(30,000 km2). Loki is covered with dark silicate lava from recent eruption, and at its eruptive height, it can produce more than 10 percent of the moon's total heat output. There are enough eruptions from Loki and other patera that the surface age of Io is current (meaning that the whole surface is freshly made); in fact, there were eight volcanic eruptions during Voyager 1 flyby alone.The record of crater-ing seen on other solar system bodies cannot be seen on Io, as they have all been covered by the volcanic eruptions. In five years of observations, Galileo saw over 16 SO2 plumes making rings on Io's surface, and six giant plumes consisting mainly of sulfur alone. During these five years, 17 percent of Io's surface was resurfaced by volcanic activity. Over 100 active volcanic centers have been found. Compared to the Earth, where every year around eight to 15 km3 of lava is erupted onto the surface in total from all volcanoes, over 500 km3 of lava is erupted onto the surface of Io every year. Io's mass is tiny, only about 1.5 percent of the Earth's mass, but it erupts over 30 times as much lava. Io is so voluminously volcanic that if it had erupted at this rate continuously over the last 4 billion years, it would have melted its mantle and crust 80 times over and erupted 40 times its own volume! This indicates a great ability of the silicate portion of Io to flow, constantly reforming a spherical planet.

This kind of volcanic recycling is unheard of in planetary science, and it requires both very high rates of heating and an unusual planetary composition. On the Earth, the main constituent of the upper mantle is the mineral olivine, and olivine is also a large component in erupted lavas. Analysis of reflectance spectra from Io indicate that the lavas are very high in magnesium; this helps explain their high temperatures, because magnesium-rich minerals have high melting temperatures in general. Surprisingly, though, the spectra indicate that the main mineral is orthopyroxene, and not olivine. This result is reminiscent of Earth's Moon, where a high percentage of its mantle is also thought to be made of orthopyroxene. One possible explanation for Io's strange composition is that some elements may have been vaporized away from hot magmas throughout Io's history, and therefore depleted from the planet as a whole (or trapped in cold areas near Io's poles). Elements likely to be vaporized are also those that lower the melting temperature of the mantle, such as sodium, potassium, and iron, and if these have been lost throughout geologic history from the planet, the temperatures required to melt the planet's interior would have risen through time.

Each day the surface temperature of Io varies between —330 and —200°F (-200 and -140°C). Voyager 1 measured 80°F (27°C) in a dark region, exceptionally warm compared to the rest of the planet. Io's surface temperatures, with the exception of volcanic hot spots, are about the same on the poles as at the equator. On Earth, the equatorial areas are hotter because of the more direct sunshine.Why is Io different? There may be more volcanic heating at the poles, or there may be materials there that hold heat more effectively, keeping the poles warm while the equator cools. To support the volcanoes that are seen, the crust has to be a few tens of kilometers thick. Io thus has another paradox: It must have a thick stiff crust to support the high volcanoes that exist, but all of its material needs to flow enough so that the moon has constant volcanic eruptions and yet remains a sphere.

Tidal stresses from Jupiter's giant, close-by mass are thought to be important in production of volcanism. While Io is held tightly in Jupiter's gravity field, the other moons that pass by Io pull it in other directions. Because of these strong and opposing pulls, the surface of Io oscillates by hundreds of meters as it rotates.This kind of wild oscillation of a solid can create significant amounts of heat through friction, and it has been thought to create the volcanic activity seen. Tidal stresses are not the final word on the source of heat for Io's eruptions, however, now that more detailed calculations of heating have been made. Io's power output from volcanism is about 2.5 W/m2.This value may be more than the energy contributed from Jupiter through tidal heating (some researchers contend that it is more than twice the energy that Jupiter contributes). It is about twice the magnitude of the heating provided by electromagnetic heating from the ion storm around Io, and it also exceeds any heat possible from radioactive decay. Io's heat output, in fact, is more than twice the Earth's.Tidal heating is still accepted as the method for creating the heat required for the extravagant quantity of volcanic activity on Io.

Some insight into the internal structure of a planet or moon can be obtained from its moment of inertia factor. The moment of inertia factor is a measure of how much force is required to speed up or slow down the body's spin (for more on moment of inertia, see the sidebar "Moment of Inertia" on page 80). Io's moment of inertia factor is 0.37685 ± 0.00035. Based on its moment of inertia, it is thought to be rocky and silicate-rich, and its average density is estimated at 1,228 lb/yd3 (3,527.8 kg/m3), making it the densest object in the outer solar system. By comparison, Europa's average density is 190 lb/ft3 (3,000 kg/m3), and ice-rich moons Ganymede and Callisto have densities of 121 lb/ft3 (1,900 kg/m3) and 114 lb/ft3 (1,800 kg/m3).The mantle of Io is probably dominated by olivine, just as the Earth's mantle is, based on density and temperature constraints. Of the Galilean satellites, only Io and Europa are thought to have olivine-rich mantles.The density of an H chondrite, thought to be one of the most primitive materials in the solar system, is 243 lb/ft3 (3,850 kg/m3), and so Io and the other outer planets have been depleted of their heavy elements.

Io's moment of inertia factor also requires that it have a core. The core of Io is thought to be a combination of iron (Fe) and sulfur (S) in two forms, either Fe and FeS, or Fe O and FeS.The iron compounds here differ only by how much oxygen (O) they have bonded with; the oxygen state of the inside of Io is not known. Sulfur makes the core material less dense, and thus allows a larger core while still matching the moment of inertia factor for the planet. Depending on the core's composition, and hence its density, Io's core may be from 38 to 53 percent of the satellite's radius.The core is thought to be overlain by a silicate mantle, and then about 50 miles (80 km) of crust, with a density of about 190 lb/ft3 (3,000 kg/m3).

Perhaps Io is still a mushy magma ocean, covered with a crust.This structure would complicate the explanation for the moon's weak magnetic field: If Io does consist primarily of a magma ocean, it might lack the internal structure thought necessary to form a magnetic field. On the other hand, a magma ocean might remain well mixed through convection, helping to explain the homogeneity of the lava compositions erupted on Io. Density calculations indicate that the mantle cannot be fully molten, though it may be partially molten. Io has become a focus of tremendous scientific attention, and it is hoped that spacecraft may send back data to help answer some of these questions.